n CAUSES OF ADVANCE OR RETROGRESSION 53 



assumption. Although he explains it as a mechanico-physio- 

 logical principle, I hold it to be a kind of striving towards a 

 goal, or teleology, in face of which the recognition of a 

 directing power conceived as personal, existing outside 

 material nature and ruling all things, would seem to me fully 

 justified. 1 



The formation of species is not completely comparable to 

 the sprouting of the twigs of a tree. It depends not only on 

 increased perfection, but just as much on deterioration and 

 simplification, and on retrogression in complexity and in 

 division of labour, i.e. upon growth, with all the changes which 

 growth can undergo through hindrance and through the 

 modifications which are imposed upon it. 



The abundance of the species which have been formed by 

 degeneration, by . retrogression, is known to every zoologist. 

 It is self-evident that their origin is to be traced to the action 

 of external conditions, that they have been produced by 

 acquired and inherited changes of growth depending on these 

 conditions. 



My own extensive researches on the variation of animals, 



1 Here I may be allowed also a remark pro domo. Nageli in the introduction 

 to his book speaks very severely of those who without any justification undertake 

 to express opinions upon the question of the origin and the evolution of organisms. 

 He claims this right exclusively for those who are physiologists by profession, 

 and counts among the non-physiologists both Darwin and Haeckel. Against such 

 a close corporation I protest. Investigators who have enjoyed a thorough 

 physiological training, and whose ideas must in consequence of this training have a 

 physiological basis and among such I Would modestly request to be numbered 

 are surely not to be excluded. But, as every one knows, outsiders have at times 

 larger ideas than the members of a corporation. Besides, we might in accordance 

 with Nageli's argument conversely inquire whether the laws of vegetable physio- 

 logy and the facts at the disposal of this limited branch of science are 

 sufficient to justify the treatment of a matter which concerns the whole 

 morphology and physiology of the animal kingdom ? At all events, the animal 

 kingdom, on account of its greater complexity and the active functions of its organs, 

 affords a wide field for the examination of questions which never come to the 

 notice of the botanist at all. For this reason, the impulse towards a more 

 scientific doctrine of evolution has come from zoologists, or at least, from men 

 zoologically trained, not exclusively from botanists. 



